The present invention relates to a powder coating composition giving an excellent metallic luster, a method for preparing it and a method for forming a film using it.
A powder coating has been increasingly applied to automobiles, household appliances, building materials, toys and the like as a low-pollution coating with no organic solvent. When the powder coating is applied to obtain a metallic finish, however, a metallic pigment is hardly aligned parallel to a substrate. Then, a color tone becomes dark and a sufficient metallic luster is not obtained.
For the preparation of the powder metallic coating, a method comprising previously kneading a metallic pigment with a resin and a color pigment by melting, spray drying or the like (a melt blend method), a dry blend method comprising mixing a resin powder with metallic flakes previously powdered by drying, a method comprising depositing a metallic pigment onto a surface of a powder coating resin by means of a brush polisher or the similar means (a bonded method) or the like have been developed (for example, see JP-51137725A, JP-82035214B, U.S. Pat. No. 4,138,511 etc.).
In the melt blend method, a metallic pigment is easily deformed during the kneading step or the step of controlling a particle size of a powder coating by grinding or the like. Then, a satisfactory appearance is not obtained. Since the metallic pigment is ground, an active surface is exposed so that risks of fire, dust explosion and the like are increased.
In the dry blend method, a metallic pigment is hardly deformed. Therefore, this method is more effective than the melt blend method in design. However, it is also difficult to align aluminum flakes parallel to a substrate by this method so that a sufficient metallic luster is not obtained. Because of the difference between a resin powder and metallic flakes in electrostatic property, an initial composition of a powder coating is not identical with a composition of a film prepared by using the powder coating. Thus, the powder coating recovered is reused with difficult.
The bonded method has a merit that an input ratio of metallic flakes in a film is constant since the metallic pigment is deposited onto a surface of a resin and therefore, a powder coating recovered without depositing onto a substrate can be reused. Although a metallic luster is easily obtained since a metallic pigment can be oriented along a surface of a resin according to this method, in practice the surface of the resin is not always aligned parallel to the substrate. Thus, the sufficient metallic luster is not obtained.
A primary object of the present invention is to overcome defects in appearance including metallic luster, brightness, gloss of a film in the prior metallic powder coating and to provide a powder coating composition meeting basic requirements as a powder coating such as an input ratio of metallic flakes, a coating operability, a recovery efficiency, a chemical resistance and the like.
These and other objects of the present invention can be achieved by a powder coating composition comprising metallic flakes having a leafing agent adsorbed on their surfaces, said metallic flakes being mixed with a thermosetting resin powder or deposited onto a surface of a thermosetting resin powder.